Validation of a subject specific 3-actuator torque-driven model in human vertical jumping.

In this study, a forward dynamic subject specific 3-actuator torque-driven model of the human musculoskeletal system was created based on measurements of individual characteristics of a subject. Simulation results were compared with experimental vertical squat jumping with and without adding weights. By analyzing kinematic and kinetic experimental data at the instant of the toe-off for the same initial conditions, it was shown that a simple computer simulation using a suitable cost function could reproduce the real task performed by humans. This investigation is the first step in a wider project that will incorporate elastic components, and that will evaluate the advantages of the individual subject approach in modeling.

How the ankle joint angle alters the antagonist and agonist torques during maximal efforts in dorsi- and plantar flexion.

The aim of this study was to assess, via an EMG bio-feedback method, the ankle joint angle effect on the agonist and antagonist torques in plantar- (PF) and dorsi-flexion (DF). The isometric PF and DF maximal voluntary contractions (MVCs) torques were measured simultaneously with surface EMG activity of triceps surae (TS) and tibialis anterior (TA) muscles in 12 young adults (mean age 27) at five different ankle joint angles. Our results showed that: (i) The coactivation level does not properly reflect the mechanical effect of the antagonist muscle, (ii) TS antagonist torque significantly altered the DF MVC-angle relationship, whereas TA antagonist torque did not influence this MVC-angle relationship in PF. The alteration of the MVC with angular position was due, in part, to the coactivation phenomenon in DF, but not in PF. Thenceforth, when investigating the torque at the ankle joint, it is necessary to take into account both agonist and antagonist torque modifications with ankle joint angle.

Differences in twitch potentiation between voluntary and stimulated quadriceps contractions of equal intensity.

This study compared the extent of twitch and M-wave potentiation (POT) between voluntary and stimulated quadriceps contractions performed at the same intensity. Sixteen healthy men completed 10-s isometric knee extensions at 40% of the maximal voluntary contraction torque under electrical stimulation and voluntary conditions. Single stimuli were delivered to the femoral nerve to evoke twitches before (PRE) and from 3 to 600 s after the end of each conditioning contraction. Changes in twitch contractile properties and M-wave characteristics were compared between the conditions. The extent of twitch peak torque POT was smaller for the stimulated (122+/-20% of PRE) than for the voluntary condition (133+/-20% of PRE). The magnitude of POT for the maximal rate of twitch torque development was also smaller for the stimulated trial. Rectus femoris M-wave amplitude was potentiated by the voluntary but not by the stimulated contraction. It was concluded that stimulated contractions resulted in smaller twitch and M-wave POT than voluntary contractions, despite equivalent torque output and duration. The spatially and temporally fixed recruitment of motor units with electrical stimulation and therefore the lower number of activated motor units compared with voluntary actions of equal intensity could explain the present findings.

Plantar flexion torque as a function of time of day.

The possible peripheral and/or central origin in the mechanisms responsible for day-time fluctuation in maximal torque of the triceps surae muscle were investigated with a special emphasis on antagonist muscle coactivation. Eleven healthy male subjects (physical education students) took part in this investigation. The electromechanical properties of the plantar flexor muscles were recorded at two different times of day: between 06:00 h and 08:00 h in the morning and between 17:00 h and 19:00 h in the evening. To investigate peripheral mechanisms, the posterior tibial nerve was stimulated at rest, using percutaneous electrical stimuli, to evoke single twitch, double twitch, and maximal tetanic contraction (100 Hz). Maximal voluntary contraction of the plantar flexors was also assessed by means of the relative electromyographic activity of respective agonist and antagonist muscles (soleus, gastrocnemius medialis, gastrocnemius lateralis, and tibialis anterior). A double twitch was delivered during maximal voluntary plantar flexion to record muscle activation (i.e., interpolated twitch technique). The coactivation level of the tibialis anterior muscle during plantar flexion was calculated. The results indicated a significant decrease in maximal voluntary muscle torque of triceps surae in the evening as compared with the morning (-7.0 %; p < 0.05). Concerning the central command, when extrapolated by the twitch interpolation technique, the decrease in mean activation level of -6.8 % was consistent with the fluctuation in torque (-7.0 %). Soleus muscle electromyographic activity (normalized to the M-wave) showed a significant decline (21.6 %; p < 0.001). Moreover, individual changes in MVC percentage were significantly related to those of normalized electromyographic activity of the soleus muscle (r = 0.688; p < 0.01). Thus, it indicated that the subject's capacity to activate the soleus muscle was affected by the time of day. The coactivation level in the tibialis anterior muscle during plantar flexion did not change significantly in the evening. Concerning peripheral mechanisms, we observed a decrease in maximal M-wave amplitude for soleus and gastrocnemii, associated with unchanged single twitch and tetanus torque. To conclude, impairment in soleus muscle central command seemed to be the mechanism in the origin of torque failure. Such information would be of importance in the investigation of day-time fluctuations in complex motor task performances implicating the triceps surae muscle.

Stability of pedalling mechanics during a prolonged cycling exercise performed at different cadences.

The aim of this study was to analyse the effect of pedalling rate on the pattern of mechanical torque application and on neuromuscular fatigue during prolonged cycling exercise. Eleven well-trained individuals performed three 1-h pedalling sessions, at 50 rev.min-1, 110 rev.min-1 and a freely chosen cadence, at an intensity corresponding to 65% of their maximal aerobic power. The mechanical torque applied on the right pedal was recorded for 30 s every 5 min while pedalling. Contractile and neural properties of the quadriceps and hamstring muscles were analysed before and immediately after each of the three pedalling sessions. The post-exercise reduction in knee extensors maximal voluntary contraction was significant (P<0.01) irrespective of the cadence, but no difference was found between cadences. The use of a particular cadence did not lead to preferentially central or peripheral fatigue. An increase in cadence resulted in greater positive and negative work generated during pedalling. The mechanical pattern was not altered during the exercise, whatever the selected cadence. The present study demonstrates that despite the occurrence of neuromuscular fatigue, trained individuals maintained a stable pedalling pattern throughout an endurance cycling exercise for cadences ranging from 50 to 110 rev.min-1.

Neuromuscular efficiency of the triceps surae in induced and voluntary contractions: morning and evening evaluations.

Variations in force and electromyographic (EMG) activities of skeletal muscles with the time-of-day have been previously described, but not for a postural muscle, submitted to daily postural and locomotor tasks. In this article, mechanical performances, EMGs, and the ratio between these parameters, i.e., the neuromuscular efficiency (NME), were measured on the triceps surae (TS) of eight subjects, two times each day, at 6:00 and 18:00 h. NME was evaluated under different experimental conditions (electrically induced contractions, reflex contractions, maximal and submaximal voluntary isometric contractions, and during a natural movement, a drop jump) to determine whether mechanisms, peripheral or central in origin, were responsible for the eventual changes in NME with time-of-day. To calculate NME in induced conditions (NMEind), a supramaximal electrical stimulus was applied to the tibial nerve, and the maximal M wave of TS (TS Mmax) and the amplitude of the twitch tension (PtMmax) in response to this electrical stimulation were quantified. TS Mmax was significantly lower in the evening (mean gain value -10.7 +/- 5.5%, p < 0.05), whereas PtMmax was not significantly modified. NMEind (PtMmax/TS Mmax) was significantly higher in the evening (mean gain of 17.6 +/- 5.8%, p < 0.05), and this increase was necessarily peripheral in origin. Secondly, maximal tendon taps were applied to the Achilles tendon in order to quantify at the two times-of-day the reflexes in response to a mechanical stimulus. The maximal reflex, TS Tmax/Mmax (%), the peak amplitude of the twitch tension associated to this tendon jerk (PtTmax), and the corresponding NME (NMEreflex = PtTmax/TS Tmax/Mmax) were not affected by time-of-day, indicating that reflex excitability did not present daytime variations when tested under these conditions. Voluntary isometric contractions were required under maximal (MVC) and submaximal (25% MVC) conditions, and the corresponding torques and TS EMG were measured. MVC was higher in the evening (mean gain: 8.6 +/- 2.7%, p < 0.05) and TS EMGmax (normalized with regard to TS Mmax) also increased in the evening but not significantly; thus, NMEMvc was not modified. At 25% of MVC, TS EMG was significantly higher in the evening (mean gain of 23 +/- 13.9%, p <0.05) and a trend for a lower NME25%MVC in the evening was observed, a result probably representative of a higher muscle fatigue state in the evening. Finally, to test the muscle capacities during a natural task, a NME index was calculated during a drop jump (DJ). The NMEDJ was defined as the ratio between jump height and mean amplitude of TS EMG (% of TS Mmax) between the drop and the jump. Both jump height and NMEDJ were significantly higher in the evening (mean gains of 10.9 +/- 4.5% and 15.7 +/- 7.4%, respectively, p <0.05). In conclusion, daytime changes in the efficiency of postural muscles seem to depend on both peripheral and central mechanisms. According to the experimental conditions, NME of the postural muscle could increase, remain constant, or even decrease in the evening, and this result may reflect reverse effects of better contractile capacities and higher fatigue state.

[The Dijon Physical Activity Score: reproducibility and correlation with exercise testing in healthy elderly subjects]. / Le Score d'activité physique de Dijon: reproductibilité et corrélations avec l'aptitude physique de sujets sains âgés.

PURPOSE: To validate a French physical activity score for current use in healthy elderly people. PATIENTS AND METHODS: A brief questionnaire was developed for healthy elderly subjects older than 72 years. Questions asked about self-assessed physical fitness (1), daily activity (2), leisure and sport activity (5) and rest time (1). Correlations were searched between the score and several parameters of physical fitness: maximal exercise stress test (peak VO(2)), six-minute walk test, 200-meter long walk test, timed up an go test. RESULTS: A total of 57 subjects were studied (mean age, 77.6 +/-3.6 years; 28 females and 29 males). The mean time to complete the questionnaire was 3.96 minutes. Reproducibility was 0.606, as determined by Spearman's coefficient. Correlations were found between the physical activity score and peak VO(2) (0.302, P <0.05) and peak power (0.257, P <0.10). CONCLUSIONS: This easily obtained physical activity score provides a new way to assess physical activity in active elderly subjects. The physical score correlated in part with maximal ability but must be validated in several diseases.

Maximal voluntary eccentric, isometric and concentric torque recovery following a concentric isokinetic exercise.

To examine neuromuscular fatigue and recovery following an isokinetic fatiguing exercise, nine active females performed a fatiguing exercise comprising of ten sets of ten maximal concentric knee extensions. Before (pre-test), five minutes (post-test), 24 h and 48 h after the fatiguing exercise, maximal voluntary eccentric (-1.05 rad x s(-1); -2.09 rad x s(-1)), isometric (0 rad x s(-1)) and concentric (1.05 rad x s(-1); 2.09 rad x s(-1)) torque were measured. In order to distinguish central from peripheral factors involved in torque decrement, activation level (twitch interpolation technique) and twitch contractile properties were recorded. During the course of the fatiguing exercise, concentric torque was significantly lower during the 3rd set than pre-test (-5.6 +/- 12.3 %) and further decreased to the 10th (-10.3 +/- 9.5 %). Eccentric and isometric torques were significantly lower during post-test than pre-test (-16.8 +/- 8.8 % at -2.09 rad x s(-1), -15.1 +/- 7.4 % at -1.05 rad x s(-1), and -10.4 +/- 5.9 % at 0 rad x s(-1); p < 0.05), while concentric torque was not significantly modified. Voluntary activation, peak twitch torque, twitch maximal rates of force development and relaxation were also significantly declined (p < 0.05) at post-test. Twenty-four hours later, all the measured parameters were close to their pre-fatigue values. The present results reveal that the best way to test concentric-induced alteration of neuromuscular function was to use stressful testing conditions, such as eccentric contractions.

Strength and power changes of the human plantar flexors and knee extensors in response to resistance training in old age.

AIM: The aim of the present study was to assess and compare the improvements of muscle strength and power induced by a 16-week resistive programme in a population of 16 older men aged 65-81 years. METHODS: Training was performed three times per week at an intensity of 80% of one repetition maximum (1RM) and consisted of both calf raise and leg press exercises. Before-, during- and after-training, maximum isometric and isokinetic torques, maximum power, 1RM, muscle cross-sectional area (CSA) and electromyographic activity (EMG) of the plantar flexors (PF) and knee extensors (KE) were examined. RESULTS: For the KE and PF, respectively, training resulted in a 29.9 +/- 4.4% (mean +/- SE) and 21.6 +/- 5.4% increase in 1RM (P < 0.001-0.01), a 19.4 +/- 4.3 and 12.4 +/- 4.7% (P < 0.001-0.05) increase in maximum isometric torque, and a 24.1 +/- 6.3 and 33.1 +/- 10.9% (P < 0.05) increase in maximum muscle power, calculated from torque-angular velocity curves. The large increase in torque and power was partly accounted by a significant increase in the CSA of the PF (5.0 +/- 0.7%) and KE (7.4 +/- 0.7%), while no significant changes in integrated EMG activity of vastus lateralis and soleus muscles, and in extrapolated maximum shortening velocity were found. After training, a significant increase in torque/CSA (10.3 +/- 4%, P < 0.05) was found for the KE but not for the PF. CONCLUSION: Hence, hypertrophy cannot alone justify the increase in torque, and other factors, such as an increase in individual fibre-specific tension (in the case of KE), a decrease in antagonist muscles' coactivation, an improved co-ordination and an increased neural drive of the other heads of quadriceps may have contributed to the increments in strength. The significant increase in muscle power seems particularly noteworthy with respect to daily activities involving the displacement of the body over time, namely, the generation of muscle power.

Plantar flexor activation capacity and H reflex in older adults: adaptations to strength training.

The purpose of this study was to investigate whether the voluntary neural drive and the excitability of the reflex arc could be modulated by training, even in old age. To this aim, the effects of a 16-wk strengthening program on plantar flexor voluntary activation (VA) and on the maximum Hoffman reflex (H(max))-to-maximum M wave (M(max)) ratio were investigated in 14 elderly men (65-80 yr). After training, isometric maximum voluntary contraction (MVC) increased by 18% (P < 0.05) and weight-lifting ability by 24% (P < 0.001). Twitch contraction time decreased by 8% (P < 0.01), but no changes in half relaxation time and in peak twitch torque were observed. The VA, assessed by twitch interpolation, increased from 95 to 98% (P < 0.05). Pretraining VA, also evaluated from the expected MVC for total twitch occlusion, was 7% higher (P < 0.01) than MVC. This discrepancy persisted after training. The interpolated twitch torque-voluntary torque relationship was fitted by a nonlinear model and was found to deviate from linearity for torque levels >65% MVC. Compared with younger men (24-35 yr), the H(max)- to M(max) ratio and nerve conduction velocity (H index) of the older group were significantly lower (42%, P < 0.05; and 29%, P < 0.001, respectively) and were not modulated by training. In conclusion, older men seem to preserve a high VA of plantar flexors. However, the impaired functionality of the reflex pathway with aging and the lack of modulation with exercise suggest that the decrease in the H(max)- to M(max) ratio and H index may be related to degenerative phenomena.

Nycthemeral variations in core temperature and heart rate: continuous cycling exercise versus continuous rest.

Circadian rhythms have formed the subject of many researches in man during bed rest or usual routine, but have been little studied during continuous and sustained physical exercise. This study deals with the influence of time of day on biological markers in competitive cyclists during continuous physical exercise versus continuous rest. Ultra-distance cyclists were studied over a 24 h period (13:00 to 13:00 h the next day) in the laboratory. The subjects were requested to maintain a constant speed (set at 65% - 70% of their maximal aerobic speed obtained during a preliminary test) on their own bicycles which were equipped with home trainers. Workload, core temperature and heart rate were monitored continuously. The same measures were also recorded while the athletes were resting awake until 13:00 h the next day. Results show that in both situations, core temperature and heart rate exhibited significant circadian variations (p < 0.001). Furthermore, during exercise, an accentuation of amplitude and mean of every rhythm (p < 0.05) with a phase lag (p < 0.05) were observed. Despite a strenuous and continuous physical exercise requiring special physiological adaptations, the rhythmic variations observed at rest persisted, which highlighted the influence of biological clocks.

Activation of human quadriceps femoris during isometric, concentric, and eccentric contractions.

Maximal and submaximal activation level of the right knee-extensor muscle group were studied during isometric and slow isokinetic muscular contractions in eight male subjects. The activation level was quantified by means of the twitch interpolation technique. A single electrical impulse was delivered, whatever the contraction mode, on the femoral nerve at a constant 50 degrees knee flexion (0 degrees = full extension). Concentric, eccentric (both at 20 degrees /s velocity), and isometric voluntary activation levels were then calculated. The mean activation levels during maximal eccentric and maximal concentric contractions were 88.3 and 89.7%, respectively, and were significantly lower (P < 0.05) with respect to maximal isometric contractions (95.2%). The relationship between voluntary activation levels and submaximal torques was linearly fitted (P < 0.01): comparison of slopes indicated lower activation levels during submaximal eccentric compared with isometric or concentric contractions. It is concluded that reduced neural drive is present during 20 degrees /s maximal concentric and both maximal and submaximal eccentric contractions. These results indicate a voluntary activation dependency on both tension levels and type of muscular actions in the human knee-extensor muscle group.

Changes in isokinetic torque and muscular activity of elbow flexors muscles with age.

This study examined the influence of aging on torque-angular velocity relationships for elbow flexion and the corresponding muscular activity levels in order to target the mechanisms involved in the eccentric muscle action in older adults. Maximal constant angular torque (CAT) at 90 degrees was measured at different angular velocities for concentric (CON; 60, 120, 180, 240 degrees s(-1)), isometric (ISO) and eccentric (ECC; -60, -120 degrees s(-1)) elbow flexor muscle actions in older (OG; 6 females and 4 males, 64-82 years) and young adult subjects (YG; 6 females, 6 males, 19-24 years) on an isokinetic dynamometer. Myoelectrical activity was quantified on biceps and triceps muscles, using the root mean square (RMS) procedure over a range of 30 degrees motion (75-105 degrees ). Absolute CAT was significantly greater (p<0.04) for YG in comparison with OG for all types of actions (CON, ECC, ISO). The only effect of gender concerned absolute strength values (p=0.00007). However, the OG showed higher (p<0.001) relative CAT values (expressed as percentage of CON 60 degrees s(-1) value) during ECC muscle action than the YG. Nevertheless, RMS values for elbow flexors were significantly (p<0.03) lower in the OG than in the YG. The antagonist (triceps) co-activation was similar for both groups. The relative ECC force preservation with aging seems to be independent of a muscular activation phenomenon.

Short-term changes in the series elastic component after an acute eccentric exercise of the elbow flexors.

We have studied the effect of a unique eccentric exercise session on the series elastic component (SEC) properties of human elbow flexors. Ten active females performed five sets of ten maximal eccentric contractions on an isokinetic ergometer. Maximal isometric (MVC) torque, and the corresponding myoelectrical activity (RMS) of the biceps brachii and the triceps brachii muscles were recorded before, immediately after, 48 h after, and 1 week after the exercise session. SEC compliance was also measured, using the in situ quick-release technique. Maximal eccentric torque declined significantly among the five sets of the exercise session (P < 0.01). The exercise session induced a significant decrease of the MVC over the post-exercise 48-h period. MVC returned to its control value 1 week after the exercise session. The neuromuscular efficiency calculated from the MVC torque:RMS ratio did not change throughout the experimental period, suggesting an optimisation of the neural drive to muscle force production capacity. The compliance values increased when force values decreased, but the slope of the relationship between both compliance and force logarithmic values was not significantly modified by the experimental procedure, indicating that the intrinsic properties of the SEC were not affected by the eccentric exercise session. The contractility-elasticity coupling was also maintained at a constant level during the recovery period. In other words, after an acute eccentric exercise, the evolution of SEC compliance was closely linked to force changes.

Time of day effects on isometric and isokinetic torque developed during elbow flexion in humans.

The aim of this study was, firstly, to confirm or refute the existence of circadian rhythms during several velocities of concentric action of the elbow flexor muscles and, secondly, to compare the characteristics of these circadian rhythms with those obtained during isometric actions. Eight volunteer subjects participated in this study. The circadian rhythms were obtained from six test sessions (TS) carried out at different times of day over 6 days with only one TS a day. During each TS, oral temperature and the torque of the muscle action were measured. The subjects made, on an isokinetic ergometer, two maximal isokinetic concentric elbow flexions at five angular velocities (60, 120, 180, 240 and 300 degrees.s-1) and at an angle of 60 degrees. Torque-angular velocity relationships, which characterised the functioning of the muscle during concentric and isometric actions, were established for the different times of day. The values of the torque recorded at each of the angular velocities presented a clear circadian rhythm. After normalisation of the torque values, no significant differences were observed among the computed characteristics of the circadian rhythms obtained at different angular velocities. Since the circadian rhythms during isometric and concentric torque were the same, the characteristics of the circadian rhythms of the musculo-skeletal system can be studied using either type of muscle action. The results indicated that torque and temperature varied concomitantly during the day. Thus, the recording of body temperature allows one to estimate the times of occurrence of maximal and minimal values in the circadian rhythm of muscle torque.

Evidence of neuromuscular fatigue after prolonged cycling exercise.

PURPOSE: The purpose of this study was to analyze the effects of prolonged cycling exercise on metabolic, neuromuscular, and biomechanical parameters. METHODS: Eight well-trained male cyclists or triathletes performed a 2-h cycling exercise at a power output corresponding to 65% of their maximal aerobic power. Maximal concentric (CON; 60, 120, 240 degrees x s(-1)), isometric (ISO; 0 degrees s(-1)), and eccentric (ECC; -120, -60 degrees x s(-1)) contractions, electromyographic (EMG) activity of vastus lateralis (VL) and vastus medialis (VM) muscles were recorded before and after the exercise. Neural (M-wave) and contractile (isometric muscular twitch) parameters of quadriceps muscle were also analyzed using electrical stimulation techniques. RESULTS: Oxygen uptake (VO2), minute ventilation (VE), and heart rate (HR) significantly increased (P < 0.01) during the 2-h by, respectively, 9.6%, 17.7%, and 12.7%, whereas pedaling rate significantly decreased (P < 0.01) by 21% (from 87 to 69 rpm). Reductions in muscular peak torque were quite similar during CON, ISO, and ECC contractions, ranging from 11 to 15%. M-wave duration significantly increased (P < 0.05) postexercise in both VL and VM, whereas maximal amplitude and total area decreased (VM: P < 0.05, VL: NS). Significant decreases in maximal twitch tension (P < 0.01), total area of mechanical response (P < 0.01), and maximal rate of twitch tension development (P < 0.05) were found postexercise. CONCLUSIONS: A reduction in leg muscular capacity after prolonged cycling exercise resulted from both reduced neural input to the muscles and a failure of peripheral contractile mechanisms. Several hypothesis are proposed to explain a decrease in pedaling rate during the 2-h cycling with a constancy of power output and an increase in energy cost.

Circadian rhythms in human muscular efficiency: continuous physical exercise versus continuous rest. A crossover study.

This study deals with the influence of time of day on neuromuscular efficiency in competitive cyclists during continuous exercise versus continuous rest. Knee extension torque was measured in ultradistance cyclists over a 24h period (13:00 to 13:00 the next day) in the laboratory. The subjects were requested to maintain a constant speed (set at 70% of their maximal aerobic speed obtained during a preliminary test) on their own bicycles, which were equipped with cyclosimulators. Every 4h, torque developed and myoelectric activity were estimated during maximal isometric voluntary contractions of knee extensors using an isokinetic dynamometer. Mesenteric temperature was monitored by telemetry. The same measures were also recorded while the subjects were resting awake until 13:00 the next day. During activity, torque changed within the 24h period (p < .005), with an acrophase at 19:10 and an amplitude of 7.8% around the mean of 70.7%. At rest, a circadian rhythm was observed in knee extensor torque (p < .05), with an acrophase at 19:30 and an amplitude of 6% around the mean of 92.3%. Despite the standardized conditions, the results showed that isometric maximal strength varied with time of day during both a submaximal exercise and at rest without prior exercise. The sine waves representing these two rhythms were correlated significantly. Although at rest the diurnal rhythm followed muscular activity (i.e., neurophysiological factors), during exercise, this rhythm was thought to stem more from fluctuations in the contractile state of muscle.

The relative contribution to the plantar-flexor torque of the soleus motor units activated by the H reflex and M response in humans.

This study proposes a method of quantifying the relative contribution to the plantar-flexor torque of soleus H and M responses evoked by tibial nerve stimulation. For ten subjects, the amplitude of the twitch produced by the H wave was plotted against the corresponding potential, for stimuli producing H without M (i.e. in the ascending portion of the H-recruitment curve). It was then assumed that the contribution of H to twitches produced by M plus H was similar for similar H waves on the curve-descending portion. Hence, the contribution of M was estimated, for the range of M waves including those accompanying H(max). The estimated mechanical contributions of H and M wave increase linearly with their potentials, the slope of the relationship being greater for H than M. The interpretation of this finding is discussed. A simple formula for assessing the contribution of the H wave to the twitch torque is given.

The effects of a prolonged running exercise on strength characteristics.

The aim of this study was to examine concentric, isometric, and eccentric strength reductions in the quadriceps muscle following a prolonged running exercise. Before and after a 2 h run (28.4+/-1.4 km) peak torque (PT) of the knee extensors at angular velocities of -120, -90, -60, 0, 60, 120, 180, 240 degrees x s(-1) using an isokinetic dynamometer, electromyographic (EMG) activity of the vastus lateralis (VL) and vastus medialis (VM) muscles and height of a counter movement jump were recorded in twelve well-trained triathletes. Counter movement jump performances decreased by 10% and PT values were all significantly lower (p < 0.01) at each angular velocity following the run. The torque loss was significantly (p < 0.01) greater under eccentric contractions (from 18 to 21%) than under concentric ones (from 11 to 14%). EMG activity (RMS) was lower in both VL and VM muscles after the 2 h run but no difference existed in RMS losses between concentric and eccentric contractions. The present results demonstrate that 1) a prolonged running exercise more greatly affects eccentric force production in the quadriceps muscle, and 2) this specificity seems to be due to an impairment of the muscular contractile mechanism rather than a modification to the neural input.

Re-examination of training effects by electrostimulation in the human elbow musculoskeletal system.

This study examines the effects of a 7 weeks sub-maximal training period of electrostimulation on the maximal isometric, concentric, eccentric voluntary torque and muscle contractile properties of the elbow flexor muscles of nine subjects. The daily program consisted of five series of six 6-s isometric actions (60 to 70% of maximal isometric voluntary action) at an elbow angle of 90 . After training the maximal voluntary isometric flexion torque increased significantly whereas the maximal voluntary isometric extension torque decreased significantly. Increases in isometric flexion torque were linked to an increase of the myoelectrical activity of the biceps brachii muscle. Under dynamic conditions flexion torque was significantly increased throughout the whole spectrum of angular velocities. These changes were accompanied by an increase in the myoelectrical activity of the agonist muscle under eccentric conditions and at two fast concentric angular velocities, without modifications of the myoelectrical activity of the antagonist muscle. The analysis of the electrical and mechanical twitches indicated that modifications of the muscle membrane electrical activity were also present at the muscle level. These results indicate that torque gains were attributed to neural adaptations and/or to a modification of the relative part of agonist and antagonist muscles in elbow flexion torque production.